Any fibrous raw material, such as wood, straw, bamboo, hemp, bagasse, sisal, flax, cotton, jute and ramie can be defibered and used in paper manufacture. Separation of the fibers of such materials is called pulping, regardless of the extent of purification involved in the process. The separated fibers are referred to as pulp, whether in suspension in water or dried to any degree. There are three principal processes for providing pulp; these being mechanical pulping, alkaline pulping and acid pulping. The principal alkaline pulping method is the Kraft process and the principal acid pulping process is the sulfite process.
Mechanical pulping was the initial process used to prepare wood pulp. In mechanical pulping, wood pulp is produced by pressing a log against a rotating grindstone or by passing woad chips or other cut-up raw materials through a refiner, which consists of counter rotating hard metal disks with grooves cut into them. Fibers separated mechanically are damaged substantially in the process and provide weaker paper. However, since such fibers retain most of the wood components, the yield of pulp per unit weight of wood is high, usually in excess of 95%.
In order to raise the quality of wood pulp, the so-called chemical methods were developed. These methods involve chipping the wood and treating the wood chips with alkaline or acidic chemicals at elevated temperatures and pressures. The lignin and part of the carbohydrates present are released during the digestion process and the pulp yield is normally about 50%. Some of the lignin is retained in the pulp, particularly in the alkaline processes, such as the Kraft process. The lignin in the pulp may be further reduced by bleaching the pulp in various sequences with chemicals such as chlorine, alkali, oxygen-gas, chlorine dioxide, hydrogen peroxide and hypochlorite to remove residual lignin and other colored impurities.
Chemical pulps have good strength properties and can have high brightness values with bleaching. These attributes, however, are obtained at the cost of low yields. This has led in recent years to development work aimed at producing mechanical pulps in high yields which have strength properties approaching those of the chemical pulps; while at the same time, retaining the opacity and bulk properties unique to the mechanical pulps. When the lignin is softened by heating the raw wood chips or other lignocelluloses with steam before and during refining under pressure, the separated fibers make significantly stronger paper than ground wood or refiner ground wood, but still weaker than chemical pulps. Such pulp is referred to as thermomechanical pulp (TMP). Still stronger pulp is obtained with somewhat lower yield by treating wood chips or other cut-up raw material with chemicals before refining. This type of pulp is called chemical thermomechanical pulp (CTMP). When larger amounts of chemicals are used, but yet insufficient to separate the fibers without refining, the pulp is called choral-mechanical pulp (CMP).
Significant developments have taken place which combine various pretreatment steps in the chemical pulping processes, whether the pulping process utilizes an alkaline material or an acid material. Thus, alkaline pretreatment prior to acid pulping is known as well as acid treatment prior to alkaline pulping.
U.S. Pat. No. 4,900,399, for example, is directed to produce chemi-mechanical pulp from lignocellulosic material wherein the material is impregnated in two stages. The lignocellulose material is first steamed and is then impregnated with an aqueous alkali. Excess unreacted alkali is removed from the chips and the chips are further impregnated with a peroxide solution. The chips are then refined by a mechanical pulping process to produce the pulp.
U.S. Pat. No. 4,404,061 to Cael describes a chemical pulping process wherein wood chips are first subjected to an acid pretreatment with potassium monopersulfate, followed by alkaline pulping by standard methods, for example, the cold soda method or Kraft process. It is indicated in the Cael patent that the pulp so produced will have a lower kappa number, lower viscosity and slightly lower yield than pulp produced from wood chips by an otherwise identical alkaline pulping process omitting the monopersulfate pretreatment.
U.S. Pat. No. 2,956,918 relates to chemi-mechanical wood pulping method wherein wood chips are pretreated with a lime slurry prior to subjecting the chips to a mechanical fiber separation operation.
U.S. Pat. No. 2,599,572 to Miller is also directed to a chemi-mechanical pulping process wherein wood chips are first impregnated with a solution of sodium hydroxide under conditions of high temperature and pressure. The sodium hydroxide solution is drained and the chips are then immersed in a water suspension of lime. The chips are cooked in the lime for a period of approximately 7 hours at approximately 250.degree. F. to render the fibers readily separable by mechanical means. The fibers are then mechanically separated.
U.S. Pat. No. 4,486,267 to Prusas is directed to a chemi-thermomechanical pulping process employing separate alkali and sulfite treatments. In the method of the '267 patent, wood chips are first impregnated with an aqueous solution containing about 3% to 10% sodium hydroxide. The chips are maintained in the sodium hydroxide solution for a period of time sufficient to permit chemical softening of the chips. The chips are removed from the sodium hydroxide solution and are impregnated in a second impregnating step with a sulfite liquor. The sulfite liquor is an aqueous solution containing about 2% to 10% by weight sodium sulfite or sodium bisulfite. The impregnated chips are cooked in the sulfite liquor at a temperature of approximately 120.degree. to 180.degree. C. for a period of time sufficient to sulfonate the lignin in the chips and to soften the chips without removing substantial amounts of lignin. The wood chips are then defibrated by passing the chips through a refining apparatus.
U.S. Pat. No. 1,880,043 to Richter relates to an alkaline chemical pulping process wherein wood chips are initially treated with a solution of a mineral acid. The wood chips are treated with a solution of mineral acid and then cooked at elevated temperature and under pressure in a solution of a suitable alkali, such as the usual Kraft or soda liquors. This process produces cellulose pulps of high alpha cellulose content and of exceedingly low pentosan content.
U.S. Pat. No. 3,617,435 to Kalisch describes an acid bisulfite wood-pulping process in which the wood chips are pretreated with an alkaline solution containing not more than about 2% by weight of formaldehyde.
U.S. Pat. No. 1,773,419 to Baker is also directed to an acid bisulfite pulping process. In the Baker process, the wood chips are subjected to pretreatment with a weak alkali solution and steam so as to render the alpha lignin insoluble in the sulfite cooking step employed at a later stage of the process. An example of weak alkali solutions are solutions of caustic soda, barium hydrate and the like. The pretreatment with a weak alkali solution takes place at temperatures of 90.degree. to 100.degree. C. for 1 to 2 hours. It is indicated that the alkali pretreatment is preferably carried on in the presence of ammonia, which creates relatively high vapor pressures at the temperature employed and forces the weak alkali solution into the pores of the wood.
The difficulty of pretreating wood with dilute aqueous sodium hydroxide is emphasized in U.S. Pat. No. 2,956,918 to Snyder. The Snyder patent states that it is impossible to treat wood uniformly with dilute aqueous sodium hydroxide. It is stated that sodium hydroxide attacks acetyl, polyuronides, pentosans, hexosans and slowly attacks lignin. If the total quantity of sodium hydroxide present is insufficient to react with all of these components, the result is that, for the most part, the surface layers are attacked and the interior mass is hardly penetrated at all. It is further indicated in the Snyder patent, that when steeping is carried out in a 5% solution of sodium hydroxide, which is in excess of that required to react with all of the reactable constituents, the outer layers of the wood become impregnated with organic sodium compounds that are colloidal, viscous substances, rather than true solutions and these substances impede or prevent complete penetration of the chips. The Snyder patent solves these perceived problems of treatment with sodium hydroxide by using a treatment of wood chips with a mixture of lime and sodium carbonate prior to mechanical refining of the wood chips.
U.S. Pat. No. 4,475,984 to Cael is directed to an alkaline pulping process wherein the lignocellulosic materials are pretreated with monoperoxysulfate to permit more ready separation of the non-cellulosic materials during the alkaline pulping step.
The delignification of aspen wood particles by treatment with hydrogen peroxide and peroxymonosulfate is discussed in an article by Springer, Tappi Journal, 73 (1) pp. 175-178 (January, 1990). The Springer article concluded that it was possible to delignify No. 40 mesh ground aspen wood particles with solutions containing peroxymonosulfate.
In an article of Minor and Springer, "Delignification of Wood Fibers with Peroxymonosulfate", Paper and Timber, 72(10), pp. 967-973 (1990) , the use of peroxymonosulfate was investigated for the delignification of wood fibers to produce paper pulp. High lignin fibbers, such as those produced by the cold-soda process and hardboard fiber process were investigated. The hardboard fiber process produces fibers with minimal physical damage by thermal softening of the middle lamella lignin, followed by mechanical refining. The resulting lignin-coated fibers are not suitable for papermaking, but fibers are provided wherein the lignin is readily accessible to oxidative reagents in solution. Neither of the foregoing articles discloses a process for delignifying wood chips as opposed to delignification of pulp or ground wood particles.
Accordingly, it is a principal object of the present invention to provide a method for pulping of wood chips and other large lignocellulosic particles utilizing an acidic oxidizing agent comprising peroxymonosulfate.
It is another object of the present invention to provide a method for chemical pulping or chemi-mechanical pulping of lignocellulosic materials wherein the method can be practiced at relatively low temperatures and relatively low or no pressure.
These and other objects of the invention will become more apparent from the following description and the appended claims.